Camshaft



March 24, 1970 E. A. THOMPSON 3,501,976

GAMSHAFT Filed Feb. 24, 1966 2 Sheets-Sheet l "uuuuuuuuuull EARL ATHOMPSON y Jf M Attorney March 24-, 1970 E. A. THOMPSON 3,501,976

CAMSHAFT Filed Feb. 24. 1966 2 Sheets-Sheet 2 EARL A THOMPSON y J rmAttorney United States Patent US. Cl. 74567 4 Claims ABSTRACT OF THEDISCLOSURE Camshafts for internal combustion engines are made of a highcarbon high chromium alloy containing from about 1.3% to about 3.1%carbon, from about 15% to about 35% chromium with the remainder iron,with or without up to about 3.5% silicon, manganese and other residuals.The alloy is cast, cooled so quickly that a relatively small number ofrelatively large primary chromium carbide particles are formed andwidely dispersed in a matrix of austenite containing a solid solution ofchromium and carbon. Then large numbers of relatively small particles ofchromium carbides are precipitated from the matrix and distributedthroughout the spaces between the large primary carbon particles leavingthe remainder of the matrix containing carbon and susceptible tosubsequent hardening. Then the casting is hardened by heating andsubsequent quenching at such temperature and at such time that thematrix is substantially converted to martensite without significantlychanging the carbide particles. The cams may be formed of a metaldifferent from the main body of the shaft, and cast thereon by forming asolid skin over previously poured metal, then remelting the skin to forma single mass of metal by pouring the second metal on the skin.

This invention relates to camshafts for internal combustion engines. Itis based in part on my discovery that certain high-carbon, high-chromiumiron alloys can be cast and subsequently heat treated to provide a novelmetallurgical structure which provides improved camshafts of surprisinghardness, durability and dimensional stability.

One of the objects of my invention is to provide an improved camshaftwhich at one stage of its manufacture is easily machinable and which ata subsequent stage has improved hardness, resistance to Wear andcorrosion and dimensional stability.

Another object is to provide an improved method of making camshafts inwhich conventional processes are combined economically to provide animproved camshaft having the qualities mentioned.

Other objects and advantages of the invention will be understood fromthe following description and claims and from the accompanying drawings,in which FIG. 1 is a side elevation of a camshaft of conventional shape,to which my invention is applied.

FIG. 2 is an enlarged end elevation, as seen from the left of FIG. 1.

FIG. 3 is a section on the plane designated by the line 33 in FIG. 2.

FIG. 4 is a photograph of a portion of a polished and etched section ofa casting showing the metallurgical structure of my invention. Thisphotograph is of metal in the condition ascast, and is magnified about1495 times. The scale line, approximately of an inch long at the bottomof the photograph represents one ten thousandth of an inch (.0001).

FIG. 5 is a photograph corresponding to FIG. 4 of the same alloy after asubsequent heat treatment.

FIG. 6 is a photograph corresponding to FIG. 4 of the same metal aftersubsequent hardening.

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FIG. 7 is a photograph corresponding to FIG. 4 of the same metal afterdrawing following hardening.

Referring to FIG. 1, 10 designates a camshaft of known external shape.This may have journals 12, 14, 16 by which the shaft is supported in theusual bearings, the usual distributor drive gear 17, and earns 18 whichoperate the usual push rods or rocker arms for actuating the enginevalves. The cams must be very hard while other parts of the shaft neednot be so hard, and preferably are machinable at some stage ofmanufacture. For example the gear is preferably machined and frontjournal 12 customarily has a number of tapped holes 20 by which thedriving and timing gear is attached, and a drilled dowel hole 22. Thismakes problems of manufacture which are solved by my invention.

I cast my improved camshaft of a high-chromium, highcarbon iron alloy.One suitable alloy contains about 2.20% carbon and about 22.5% chromium.This alloy may also contain about 1.60% silicon and about .90%manganese. The silicon may be added to make the alloy easier to pour.The manganese combines with any sulphur which may be present in thematerial of which the alloy is made. Also the manganese may improve thehardenability of the matrix of the alloy upon quenching. Ordinarily,such alloys are made from available ingredients including scrap or pigiron of uncertain analysis so that the resulting alloy may containresidual quantities of copper, nickel, molybdenum and other metals. Asone example an analysis of one batch of my preferred alloy showed 2.20%carbon, 1.60% silicon, .90% manganese, 22.5 chromium, and residuals of.25 copper, 31% nickel and .17% molybdenum.

The silicon, manganese and the residuals amount to about 3.25%, and Ibelieve that these do not importantly affect the final metallurgicalstructure, for the purposes of my invention. Consequently, alloyscontaining them come within my definition of an alloy having statedranges of carbon and chromium, and having the remainder iron.

The camshaft may be cast in any suitable mold which is cooled asexplained below. I have discovered that alloys of the compositionmentioned above, or of the ranges of composition disclosed herein, canbe given a new and improved metallurgical structure by cooling quicklyafter pouring, and that this new metallurgical structure can be treatedto provide new, surprising and very desirable properties. As one examplea melt having the proportions of ingredients to provide the alloy of thecomposition set forth above was poured at about 2750 F. This particularalloy has a liquidus temperature of about 2399 F. and a solidustemperature of about 2270 F., as determined by the Leeds and Northrupcarbon determinator.

FIG. 4 is a photograph of a portion of a casting which has been cooledaccording to my invention. The temperature of this casting has beenreduced from the liquidus to the solidus so quickly that two things havehappened. One is that the usual formation of primary chromium carbideparticles has been arrested, so that the chromium carbide particlesformed are fewer in number and smaller than they would be if the metalhad cooled slowly. Evidence of this is that the matrix has remainedessentially nonmagnetic austenite. If the casting had cooled slowly,austenite would not be formed. The other thing that has happened is thatthe matrix contains large amounts of chromium and carbon in solidsolution. Evidence of this is the subsequent formation of very finechromium carbide particles during subsequent heat treatment. If thecasting had cooled slowly the carbon and chromium now remaining insolution would have precipitated out as primary carbides. The primarychromium carbides shown in FIG. 4 are very small, much smaller than ifthe casting had cooled slowly, also they are more widely dispersed. Thelargest primary carbide particle visible in FIG. 4, measured in inchesis about .00135 long, and in a representative area .001 square there areabout 17 primary carbide particles. The large dark particles shown arewhat is generally called chromium carbides. Among such chromium carbidesCr C and Cr C have been identified. It is also possible for iron toreplace some of the chromium to form complex or mixed chromium ironcarbides such as (FeCr) C. All of these come within the definition ofchromium carbides as that term is generally understood and used herein.The spaces, relatively large with reference to the carbides, areaustenite and substantially nonmagnetic. The hardness of the alloy ascast is about 44 Rockwell C.

The metal shown in FIG. 4 was cast in a thin shell mold of silicon sandbound with 3% phenolic resin binder.

The metal was poured at about 2750 F. into molds at room temperature.The thickness of the metal cast and the cooling characteristics of themold were such that when the casting had cooled below the solidustemperature for this particular alloy, that is about 2270 F. themetallurgical structure shown in FIG. 4, and described above, wasformed.

I have found that faster cooling forms even smaller and fewer primarychromium carbide particles. The thickness of the metal influences therate of cooling and this influences the metallurgical structure andproperties of the cast metal, not only as cast, but in subsequenttreatment. For example a part of thin cross section such as the mainshaft section cools faster than the thick journals. There is animportant and discernible difference in the appearance and properties ofthe metallurgical structures of the two portions as cast. After finalhardening, as dis- .closed below, a thicker cast (slowly cooled) issofter than a thinner casting (quickly cooled). For example a test willhave an ultimate hardness of about 60 Rockwell C, whereas a body having.160 thickness and cooled as described will have a final hardness ofabout 63 Rockwell C.

I may affect the cooling in other ways. Since a thick section cools moreslowly than a thin section it may be necessary to mold thicker sectionsin zircon sand, for example, which cools the casting faster than siliconsand. Alternatively, chills may be placed in the mold to accelerate thecooling of certain thick parts of a casting, or I may use a permanentmold, water cooled. If the metal cools too slowly the casting will notonly be too hard, but it cannot be satisfactorily heat treated so as tobe machinable.

The important thing is that the temperature of the metal must be reducedfrom the liquidus to the solidus so quickly that only relatively smallnumbers of very small chromium carbides can form, and that they will beformed in an austenite matrix which has large intercarbide spaces inwhich larger numbers of still smaller chromium carbides can beprecipitated upon reheating, while leaving the matrix containing carbonand in a condition which can be hardened. FIG. 4 shows a typicalstructure, which has properly cooled according to my invention.

After cooling the casting was heat treated as follows. Its temperaturewas slowly raised from room temperature to about 1600 F. The timerequired was three hours. It was held at 1600 F. one hour. It was cooledto about 1400 F. during the next 40 minutes. It was cooled to about 1300F. during the next hour. Total time 5% hours.

FIG. 5 shows a casting after this treatment. It shows that the chromiumcarbides of FIG. 4 have not changed significantly. The interstices orintercarbide spaces in the previously austenitic matrix are nowsubstantially filled with a dispersion of very small precipitatedchromium carbides, having a representative size of the order of about.000018 (18 millionths of an inch). In a representative area .0001square there are about 13 of these very small particles, or about 1300particles in the .001 square containing 17 primary carbide particles.Thus although the primary chromium carbides in FIG. 4 are very small (alarge one being of the order of a thousandth of an inch long) they areof the order of from 50 to times as large as the smaller carbides formedin the reheating process. The hardness after reheating was from 27 to 33Rockwell C.

I do not know the exact nature of the matrix after reheating, shown inFIG. 5. It is magnetic. It contains carbon, so that it can be hardenedby subsequent heat treatment which appears to convert the matrixessentially to martensite having properties typical of tool steel.

In the foregoing heat treatment the time required is a function oftemperature, a lower temperature requiring a longer time. Also the timeand temperature of this reheating step influences the amount of carbonleft in the matrix and so affects the subsequent hardenability of thealloy, when hardened as disclosed below.

This particular combination of carbide particles and the characters ofthe matrix in the two conditions appear to make possible themachinability at one stage of my invention and the hardness at asubsequent stage, combined with the surprising dimensional stability andother properties I have observed.

After the foregoing reheating treatment the parts can be machined easilyand economically with high speed steel tools and surprisingly the partscan be ground to the exact final shape and desired dimensions. Forexample the holes 18 and 20 can be drilled, and the holes 18 tapped andthe diameters of the journals and the cams are ground to the exactfinished size.

Thereafter the shaft may be hardened by holding at a temperature abovethe critical temperature at which the matrix changes back into austeniteand well below the melting point, followed by quenching. The time is afunction of temperature, lower temperature requiring longer time.'Forexample the part may be held at about 1750 F. for about twenty minutes,then oil quenched. FIG. 6 shows a casting which has been cooled, thenreheated, then hardened as above described. The Rockwell C hardness isabout 63 to 65. The two sets of chromium carbide particles have remainedunchanged. The matrix has been essentially converted to martensite.

My invention makes possible grinding before hardening so close to finalsize that the minimum amount of material need be removed in the finalgrinding step. In the case of articles which are acceptable withintolerances as large as .0001 (one hundred millionths) of an inch, I cangrind to final size before hardening. This is of great advantage inmanufacturing.

After hardening, the part may be drawn by holding it at a temperaturehigher than it will ever work in service, for example of about 3750 F.,for about one hour. The hardness drops about 1 point Rockwell C and thestructure is as shown in FIG. 7, with the alloy discussed above.

The advantages of the invention are realized while varying theproportion of the ingredients of the alloy within the limits statedherein. For example I may use carbon up to 2.35 and chromium up to27.00% without significantly changing the characteristics of the alloyfrom those of the preferred analysis given above, for my purposes.

Increasing the proportion of carbon within certain critical limits tendsto increase the final hardness and hence wear resistance of thecamshaft. More carbon is required in articles having a thick section,because due to slower cooling, more carbon is combined with chromium,which has a very high aflinity for carbon. If more carbon were not used,the matrix would be so depleted that it could not be hardenedsatisfactorily. More carbon than about 2.95% appears to render thearticle impractically diflicult to machine although in some instances Ican use up to about 3.10% carbon, particularly with high percentages ofchromium, increasing the proportion of chromium within a wider range ofcritical limits tends to increase the corrosion resistance and reductionof the chromium content below about 15% appears to reduce the corrosionresistance undesirably. Increasing the proportion of chromium beyondabout 30% appears to have no important effect on either wear orcorrosion resistance, except with very high carbon percentages (above3.10% for example) and increase of chromium beyond about 35% appears tohave no advantage, and may even be undesirable. There is a desirablerelationship between the amounts of carbon and chromium to have thedesired effects because one part carbon will combine with about tenparts chromium. Therefore higher proportions of chromium require higherpercentages of carbon so as to leave in the matrix, after the reheatingstep, enough carbon not combined with chromium, to harden the matrixsatisfactorily in the hardening step discussed above.

For example with my preferred alloy first mentioned, the processesdescribed appear to leave about 1.10% of carbon in the matrix after thefirst reheating step (in which the smaller carbide particles areformed). Then when the part is hardened as described, the matrix appearsto contain no free carbon and is hardened to have properties resemblingthose of tool steel or 52100 steel. Measurements of properties of thecast and hardened alloy exceed those of steel. For example, a sample ofthe preferred alloy, cast and treated as above described showed atransverse bending stress of 693,000 pounds per square inch. From thisthe modulus of elasticity is calculated at 39,000,- 000. The modulus forsteel is about 29,000,000.

Many of the advantages of the invention are present in a range of carbonbetween 1.70% and 2.85% while using a range of chromium between and 27%.

In articles having different parts requiring different hardness, I findit of advantage to use an alloy having the general characteristicsdescribed above but being ever harder and hence even more wearresistant. In such case I may use a carbon content of about 3.10% andmay use this with a chromium content varying between about 30% and about35%. This provides an extremely hard, wear resistant material. It isdifficult to machine by cutting tools, and although it is difficult togrind I have found that by confining this material to the parts on theshaft which need not be machined I can satisfactorily machine the otherparts. This is partly due to my improved casting process which permitscasting of two different metals Within very small tolerances, andconfines the extremely hard alloy to parts which need not be machined,making it possible for me to make a camshaft to finished size with aminimum of grinding. It is also due in part to the unusual dimensionalstability of the material which makes it possible to grind to closetolerances before the hardening step of the manufacturing processdescribed above, and to finish grind by removal of the minimum amount ofmaterial.

An example of such article is disclosed in my patent in Great BritainNo. 991,513, published May 12, 1965, the disclosure of which isincorporated herein by reference with the same effect as if quotedcompletely herein. In such case the hard alloy containing about 3.10%carbon and about 27 to 30% chromium is confined to the portion of theshaft between the journals 12 and 16. One of the other alloys disclosed,such as that containing 2.2% carbon and 22.5% chromium is cast in themold to form the journal 12, and while this is still molten the 3.1%carbon alloy is cast in the same mold to form the main body of theshaft, up to but not including the gear 17. This is done by the methoddescribed in my British patent. Also while the upper surface of theshaft near the gear 17 is still molten the gear 17 and journal 16 arepoured into the mold, for example of the 2.2% carbon alloy, in the samemanner. After the shaft has cooled, there is a single integral castinghaving its ends of one metal autogenously joined to the center sectionat bonding zones 24 as more fully disclosed in the British patent.

After the entire casting has been softened, as described above the endjournals and gear may be machined and ground practically to final size,and the cams and center journals can be brought to final size withminimum grind- C Si Cu Mn Cr Ni Mo Example:

I 2.20 1. 60 25 22. 5 31 17 II 1.70 .81 .09 .60 15.0 .19 .03 III 2.68 1.34 13 24. 0 30 11 IV 2. 85 1. 47 13 1.01 27. 00 .37 17 V 3.10 1. 7659 1. 29 25. 7 .48 .34

I claim as my invention:

1. A camshaft for an internal combustion engine comprising incombination a shaft having a main portion and having a second portionwhich requires machining during manufacture, the main portion being castof one metal and the second portion being cast of another and differentmetal, the metals being autogenously joined by a connect ing zone whichhas the properties of a connecting zone which has been formed by pouringmolten first metal into a mold, then while the main body of the firstmetal is molten forming on its surface a nonliquid barrier composed ofthe first metal which prevents the flow of molten metal therethrough,then remelting the barrier by pouring molten second metal onto thebarrier to form a single body of molten metal, and then cooling themolten metal to form a casting, the second portion being formed of aniron alloy containing from about 2.2% to about 2.35% carbon and fromabout 22% to about 27% chromium with the remainder iron, the mainportion of the shaft being formed of an alloy containing about 3.10%carbon and about 25% chromium with the rest iron, each of the ironalloys having a minimum hardness of about 61 Rockwell C and having arelatively small number of relatively large primary chromium carbideparticles distributed in a matrix of martensite and having a relativelylarge number of relatively small precipitated chromium carbide particlesdistributed throughout the matrix between the large primary carbideparticles.

2. A camshaft for an internal combustion engine having a main camshaftportion formed of one iron alloy composition and a second portion formedof a second different iron alloy composition for attachment of a drivingdevice, the main portion being cast of an iron alloy containing betweenabout 1.3% and about 3.1% carbon and between about 15% and about 35%chromium with the remainder iron and the second portion being cast of aniron alloy containing between about 1.7% and about 2.85% carbon andbetween about 15% and about 27% chromium, with the rest iron.

3. A camshaft for an internal combustion engine comprising incombination a shaft having a main portion and having a second portionwhich requires machining during manufacture, the main portion being castof one metal and the second portion being cast of another and differentmetal, the metals being autogenously joined, the second portion beingformed of an iron alloy containing from about 2.2% to about 2.35 carbonand from about 22% to about 27% chromium with the remainder iron, themain portion of the shaft being formed of an alloy containing about3.10% carbon and about 25% chromium with the rest iron, each of the ironalloys having a minimum hardness of about 61 Rockwell C and having arelatively small number of relatively large primary chromium carbideparticles distributed in a matrix of martensite and having a relativelylarge number of relatively small precipitated chromium carbide particlesdistributed throughout the matrix between the large primary carbideparticles.

4. A camshaft for an internal combustion engine comprising incombination a shaft having a main portion and having a second portionwhich requires machining during manufacture, the main portion being castof one metal and the second portion being cast of another and differentmetal, the metals being autogenously joined by a mixture of the twometals in which mixture the properties of the first metal diminishprogressively toward the second metal, and the properties of the secondmetal progressively diminish toward the first metal, the second portionbeing formed of an iron alloy containing from about 2.2% to about 2.35%carbon and from about 22% to about 27% chromium with the remainder iron,the main portion of the shaft being formed of an alloy containing about3.10% carbon and about 25% chromium with the rest iron, each of the ironalloys having a minimum hardness of about 61 Rockwell C and having arelatively small number of relatively large primary chromium carbideparticles distributed in a matrix of martensite and having a relativelylarge number of relatively small precipitated chromium carbide particlesdistributed throughout the matrix between the large primary carbideparticles.

References Cited UNITED STATES PATENTS 1,245,552 11/1917 Becket 751261,582,883 4/1926 Rich 12390 2,015,991 10/1935 Breeler 123-188 2,051,4158/1936 Payson 123188 X 2,127,245 8/1938 Breeler 123188 2,199,096 4/1940Berglund 14835 X 12/1956 Fugua et al. 1483 X OTHER REFERENCES Chromiumin Cast Iron, Electro Metallurgical Co., 1939, pp. 2937 and 42 reliedon.

Kinzel et al.: Alloys of Iron and Chromium, vol. II, 1940, McGraw-HillCo., Inc., New York, N.Y., pp. 182, 183, 230-235, 244-249 and 258 reliedon.

CHARLES N. LOVELL, Primary Examiner

